1. Field of the Invention
The invention relates to a method for measuring the distance between a workpiece and a machining head of a laser machining apparatus by means of which workpieces can be welded, cut or otherwise machined.
2. Description of the Prior Art
Laser machining apparatus usually comprise a laser radiation source, which may be, for example, a CO2 laser, a fibre laser or a disc laser. A laser machining apparatus additionally includes a machining head, which focuses the laser radiation, generated by the laser radiation source, in a focal spot, and a beam delivery means, which delivers the laser radiation, generated by the laser radiation source, to the machining head. The beam delivery means in this case may comprise optical fibres or other optical waveguides, and/or one or more deflecting mirrors having planar or curved faces. The machining head may be attached to a movable robot arm or to another positioning device that enables three-dimensional positioning. The laser radiation source in this case is frequently disposed at a greater distance from the machining head or from a positioning device carrying the machining head.
Usually, the workpieces to be machined are positioned in relation to the machining head by means of handling devices. By means of the robot, the machining head is then guided over the stationary workpiece, at a distance of a few millimeters. At the same time, process gas flows out of the machining head, which process gas, depending on the machining operation, reacts chemically with the material or merely performs the function of removing residues, produced during the machining operation, from the machining site.
In the use of such laser machining apparatus, it is difficult to position the focal spot in an exact manner on the surface of the workpieces to be machined, the diameter of which focal spot is usually between 100 μm and 500 μm in the case of welding work, and may be 20 μm and less in the case of cutting work. It is ideal if the focal spot is tracked in a process of feedback control of the actually existing spatial arrangement of the workpieces. For this purpose, the actual spatial arrangement of the workpieces to be machined, relative to the machining head or to another reference point, is measured in real time during the laser machining operation.
For the measurement, the machining location, for example, can be observed by means of a camera, which captures a 2D projection of the workpieces. However, if the beam path of the camera is coaxial with the laser radiation, as is known in the prior art, only a lateral offset, along the X and Y directions, can be measured with precision, but not the distance of the workpiece in relation to the machining head, along the Z direction. A high process quality requires measuring accuracies in the Z direction that are in the order of magnitude of about 400 μm for welding work and in the order of magnitude of about 100 μm for cutting work.
Further known measuring methods are light-section methods and triangulation methods. Capacitive sensors are also used, especially for distance measurement, insofar as the workpieces have a sufficiently high electrical conductivity.
In addition, the use of optical coherence tomographs, OCT, was also proposed some time ago, for the purpose of measuring distance during laser machining, cf., in particular, EP 1 977 850 B1, DE 10 2010 016 862 B3 and DE 10 2012 207 835 A1. Optical coherence tomography makes it possible to effect highly precise measurement of distance, and even to generate a 3D profile of the scanned surfaces if the measuring beam is guided over the surfaces in the manner of a scanner. Moreover, unlike capacitive sensors, coherence tomographs allow the distances in relation to non-metallic materials, such as fibre-reinforced plastics, to be measured.
It has been found, however, that the measurement of distance by means of coherence tomographs, which is highly promising per se, does not deliver reliable measurement results under all conditions.